[dune-pdelab] Variadic Templates

[Jö Fahlke]

Am Fri, 10. Jun 2011, 20:41:43 +0200 schrieb Christian Engwer:
> I remember Jö describing a SFINAE construct to select a class,
> depending on the presence of a certain typedef in an other class. Not
> sure if this might help.

That would probably be something like

  template<class T>
  struct AlwaysVoid { typedef void Type; };

  template<class T, class Dummy = void>
  class SomeClass {
    // Stuff that works without T::Foo
  };

  template<class T>
  class SomeClass<T, typename AlwaysVoid<T>::Type> {
    // Stuff that works with T::Foo
  };

or you can make a predicate and use enable_if

  template<class T, class Dummy = void>
  class HasFoo : public false_type {};

  template<class T>
  class HasFoo<T, typename AlwaysVoid<T>::Type> : public true_type { };

  template<class T, class Dummy = void>
  class SomeClass {
    // Stuff that works without T::Foo
  };

  template<class T>
  class SomeClass<T, typename enable_if<HasFoo<T>::value>::Type> {
    // Stuff that works with T::Foo
  };

I think the alternative approach of using a bool instead of a class for Dummy
(which would allow to use Dune::AlwaysTrue) had a drawback, something along
the lines of "non-trivial integral expression in a specializations template
argument list".

Bye,
Jö.

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